The invention relates to a process for continuous determination of the optical layer thickness of coatings, which are applied on both sides of the spherical surfaces of concave convex lenses having different spherical radii R1 and R2.
There exist photographic methods for measuring the thickness of layers deposited by means of vapor on glass panes or the like. The DE-OS 36 27 232 describes a photometer, wherein a chopper produces a measurement phase, a reference phase and a dark phase, with the result that these different phases are offset in time so that there can be a single detector for all of the phases. There is also a first and a second light guide, between whose one respective end there is arranged the object to be measured. Furthermore, there is a third light guide, whose one end is located opposite the detector and whose other end is connected to a chopper, which is also connected to the other end of the first light guide. At the same time there is the drawback that there must be several light sources.
In J. Roland Jacobsson: PROCEEDINGS, volume 652, Thin Film Technologies II, 1986, page 24, the functional relationships between the reflection and the optical layer thickness are described. The studies discuss, among other things, layer systems that are constructed by applying several alternate layers of TiO2 and MgF2 on a substrate. These functional relationships can be calculated theoretically for different layer materials in accordance with the universal law of optics on regularity.
The invention is based on the problem of providing a process for continuous determination of the optical layer thickness of coatings, which are applied on both sides of concave convex lenses. Furthermore, this process is relatively simple to carry out, with the result that there is no need for standard measurements at standard substrates. The process ought to be advantageous to carry out even if the concave convex lenses exhibit layer systems comprising several different layers.
The problem, on which the invention is based, is solved with a process for continuous determination of the optical layer thickness of coatings, which are applied on both sides of the spherical surfaces of concave convex lenses with different spherical radii R1 and R2. In this inventive process a ray of light is beamed eccentrically during the coating process at each concave convex lens, and the reflection or transmission at the convex spherical surface and at the concave spherical surface is continuously measured with photodiodes, and the respective optical layer thickness is determined from the functional relationship between the reflection or the transmission and the optical layer thickness. The optical layer thickness is defiried as the product of the geometric layer thickness and the respective index of refraction, based on the selected wavelength. Uni-layered or multi-layered layer systems can be used as the coating. In the process a light ray is used that exhibits a width ranging from 0.1 mm to 5 mm. The functional relationship between the reflection or the transmission and the respective optical layer thickness is well known or can be calculated beforehand so that the optical layer thickness can be determined from the calculated values of the reflection or the transmission. It has been demonstrated surprisingly that the optical layer thickness on both sides of the concave convex lens can be determined and monitored continuously during the coating process. Since only one light source is required, it is advantageous that there is no need for standard measurements at standard substrates. This feature is especially advantageous when the convex spherical surface and the concave spherical surface are provided with different layers having different layer thicknesses as the layer systems, and both sides are coated simultaneously.
A preferred embodiment of the invention comprises that a light ray is beamed at the peripheral area of the concave convex lens. The peripheral area is defined as that outer area that extends in a width ranging from 1 to 12 mm from the outer edge of the concave convex lens to its center. In this manner the reflection can be determined in an especially advantageous manner, since the difference between the angle of reflection at the convex spherical surface and the angle of reflection at the concave spherical surface is larger in the peripheral area of the concave convex lens than in the immediate vicinity of the center of the concave convex lens. In this manner the reflection at both spherical surfaces of the concave convex lens can be determined separately by means of two separate photodiodes. The immediate result is the optical layer thickness on both spherical surfaces. If, in contrast, the ray of light is beamed precisely at the center of the concave convex lens, a feature that is not provided in accordance with the invention, then the reflections at both spherical surfaces of the concave convex lens have the same direction. Hence a separate determination of the reflection could be done only with difficulty. However, the latter is an absolute prerequisite for successfully monitoring the coating process. Thus, there is the advantageous possibility of determining separately the reflection, when a ray of light is beamed unilaterally at the peripheral area of a concave convex lens and when the radii of curvature of both spherical surfaces are different.
According to another preferred embodiment of the invention, the ray of light is split with a chopper or guided through a narrow band filter prior to reaching the photodiodes. A chopper disk can be used, for example, as the chopper. This feature enables an advantageous continuously pulsed impingement of the light beam on the concave convex lens, a feature that makes it easier to evaluate the measurement results, insofar as the coating process provides an intense, intermittent background glow, which the photodiode detects as a far greater disturbing factor than the reflection of the ray of light. The background glow usually has a negative effect on an accurate evaluation of the measurement results, insofar as the ray of light is guided unsplit to the concave convex lens. Thus the negative effects of the background glow can be avoided.
Another preferred embodiment of the invention provides that the ray of light is guided through a beam splitter prior to impinging on the concave convex lens, and the intensity of one part of the ray of light is determined with another photodiode. Thus it is possible to monitor any fluctuations, which could falsify the measurement results, when the light is being emitted from the light source.
Another embodiment of the invention provides that the ray of light is guided through a deflecting mirror that is used as a beam splitter. With the use of the deflecting mirror the ray of light can be split especially advantageously into its desired intensity.
According to another preferred embodiment of the invention, white light is used as the ray of light. White light contains in the visible range all of the wavelengths in the range from 390 nm to 770 nm. At the same time it is advantageous that one can adjust the measurement of the reflection or transmission to specific wavelengths, which owing to the method of measurement are especially advantageous to measure.
Another preferred embodiment of the invention comprises that the ray of light is guided perpendicularly onto the convex spherical surface. In this manner the reflection is especially easy to determine, especially when there is, for example, only one small vacuum chamber as the coating chamber, and the angles of reflection can be found only with difficulty for spatial reasons. This is the case especially with coating systems, where only a small number of concave convex lenses can be coated simultaneously.
Another embodiment of the invention comprises that a ray of light having a diameter ranging from 0.1 mm to 2 mm is used. In this range the measurements of the reflection or the transmission can be conducted almost without errors.
According to another preferred embodiment of the invention, eyeglass lenses are used as the concave convex lenses. The eyeglass lenses can be made of glass or plastic. Since in the coating process of eyeglass lenses the respective layer thickness of the coatings has to be set very precisely, eyeglass lenses constitute especially advantageous concave convex lenses for the proposed process.
Another preferred embodiment of the invention provides that the ray of light is guided via an adjustable mirror to a photodiode. With the use of adjustable mirrors it is easy to position the photodiodes for the purpose of beaming the ray of light at said diodes, a feature that increases the quality of the measurement results.
Another preferred embodiment of the invention comprises that the reflection or the transmission at the convex spherical surface or at the concave spherical surface is measured continuously with position sensitive detectors that are provided as the photodiodes. The use of position sensitive detectors as the photodiodes also increases the measurement accuracy of the process in an advantageous manner.
In another preferred embodiment of the invention the change in direction of the ray of light, reflected at the convex spherical surface, is measured continuously and compensated for with a control system. Owing to the thermal effects on the concave convex lenses to be treated in the coating chamber, it is possible that the radii of curvature of the convex spherical surface or the concave spherical surface can be slightly altered. The consequence is that there is simultaneously a change in the direction of the respective reflected ray of light, which can have a negative effect on the measurement of the reflection or the transmission. Therefore, it is advantageous to monitor continuously this change in direction, which can be done with a simple control circuit. Then any discrepancies in the change in direction can be corrected immediately. At the same time a measurement and control apparatus can be used that is connected directly to an adjusting unit. This feature increases the accuracy of the process for determining continuously the optical layer thickness.
Another preferred embodiment of the invention comprises that the ray of light is emitted from a laser diode that is provided as the light source. In this manner the cost of components can be saved. Thus, for example, there is no need to provide a chopper. Moreover, the arrangement of the laser diodes requires very little space, a feature that is also advantageous.
The invention is explained in detail and as an example in the following with reference to the drawings (FIG. 1 to FIG. 3).